The present invention relates to filters for the removal of particulate material from diesel engine exhaust streams, and more particularly to a porous ceramic diesel exhaust filter of improved resistance to melting and thermal shock damage under conditions encountered in diesel exhaust systems.
Diesel traps have proven to be extremely efficient at removing carbon soot from the exhaust of diesel engines. The most widely used diesel trap is the wall-flow filter which filters the diesel exhaust by capturing the soot on the porous walls of the filter body. The wall-flow filter is designed to provide for nearly complete filtration of soot without significantly hindering the exhaust flow.
As the layer of soot collects on the surfaces of the inlet channels of the filter, the lower permeability of the soot layer causes a pressure drop across the filter and a gradual rise in the back pressure of the filter against the engine, causing the engine to work harder, thus affecting engine operating efficiency. Eventually, the pressure drop becomes unacceptable and regeneration of the filter becomes necessary. In conventional systems, the regeneration process involves heating the filter to initiate combustion of the carbon soot. Normally, the regeneration is accomplished under controlled conditions of engine management whereby a slow burn is initiated and lasts a number of minutes, during which the temperature in the filter rises from about 400-600xc2x0 C. to a maximum of about 800-1000xc2x0 C.
The highest temperatures during regeneration tend to occur near the exit end of the filter due to the cumulative effects of the wave of soot combustion that progresses from the entrance face to the exit face of the filter as the exhaust flow carries the combustion heat down the filter. Under certain circumstances, a so-called xe2x80x9cuncontrolled regenerationxe2x80x9d can occur when the onset of combustion coincides with, or is immediately followed by, high oxygen content and low flow rates in the exhaust gas (such as engine idling conditions). During an uncontrolled regeneration, the combustion of the soot may produce temperature spikes within the filter which can thermally shock and crack, or even melt, the filter. The most common temperature gradients observed are radial temperature gradients where the temperature of the center of the filter is hotter than the rest of the substrate and axial temperature gradients where the exit end of the filter is hotter than the rest of the substrate.
In addition to capturing the carbon soot, the filter also traps metal oxide xe2x80x9cashxe2x80x9d particles that are carried by the exhaust gas. These particles are not combustible and, therefore, are not removed during regeneration. However, if temperatures during uncontrolled regenerations are sufficiently high, the ash may eventually sinter to the filter or even react with the filter resulting in partial melting.
It would be considered an advancement in the art to obtain a filter which offers improved resistance to melting and thermal shock damage so that the filter not only survives the numerous controlled regenerations over its lifetime, but also the much less frequent but more severe uncontrolled regenerations.
The present invention provides porous particulate exhaust filters offering improved configurations that are significantly more resistant to thermal cracking and melting damage under typical diesel exhaust conditions than current filter designs. At the same time the inventive filters have fewer temperature gradients and fewer hot spots during the regeneration process. For the purposes of this description, hot spots are region the filter which reach temperatures so high that there can develop localized melting and/or cracking of the filter in such regions.
In particular, the invention provides a ceramic filter for trapping and combusting diesel exhaust particulates which includes a honeycomb filter body having a plurality of parallel cell channels traversing the body from a frontal inlet end to an outlet end thereof, in which a portion of the cell channels are plugged in a non-checkered pattern, and the remaining cell channels are plugged in a checkered pattern.
For the purposes of the present description a checkered pattern refers to an area at either face of the honeycomb in which every other cell is plugged; a non-checkered pattern refers to a pattern other than a checkered pattern.
In one embodiment the diesel exhaust particulate honeycomb filter includes a frontal inlet end and an outlet end, a matrix of thin, porous, intersecting vertically extending walls and horizontally extending walls, which define a plurality of cell channels extending in a substantially longitudinal and mutually parallel fashion between the frontal inlet end and the outlet end; the frontal inlet end includes a first section of cells plugged along a portion of their lengths in a non-checkered pattern and a second section of cells plugged in checkered pattern, the first section of non-checkered plugged cells being smaller than the second section of checkered plugged cells. Preferably, the cells are end-plugged and the first section of non-checkered end-plugged cells forms a circular pattern which is concentric with the frontal inlet end. If the frontal inlet end has a diameter Dff and the first section of non-checkered end-plugged cells has a diameter dfs, then dfs is between xc2xd(Dff) to xc2xc (Dff), and more preferably less than xc2xc (Dff). Up to three-fourth of the cells of the first section may be unplugged. Alternatively, up to one-half of the cells of the first section may be unplugged. Alternatively, up to one-fourth of the cells of the first section may be unplugged.
In another embodiment the diesel exhaust particulate honeycomb filter includes a frontal inlet end, an outlet end, and a plurality of cell channels extending from the frontal inlet end to the outlet end, the cell channels having porous walls; part of the total number of cell channels are end-plugged in a checkered pattern either at the frontal inlet end or at the outlet end, and the remaining cell channels have internal plugs.
In another embodiment the diesel exhaust particulate honeycomb filter includes a frontal inlet end, an outlet end, a group of cell channels extending from the frontal inlet end to the outlet end which are end-plugged in a checkered pattern either at the frontal inlet end or at the outlet end, and another group of partial cell channels which extend partially from the frontal inlet end into the honeycomb filter body, the partial cell channels being unplugged at the frontal inlet end.
The invention is also a method of regenerating a filter by providing a ceramic filter for trapping and combusting diesel exhaust particulates which includes a honeycomb structure as described above and heating the structure to a temperature which would facilitate the combustion of carbon soot.